Vacuum may be used to assist a driver applying vehicle brakes. The vacuum operates on a diaphragm that helps to move a rod that is linked to a brake pedal and that increases hydraulic pressure in brake lines to apply vehicle brakes. The vacuum acts as a force over the area of the diaphragm to assist the driver applying the brakes. Consequently, if the vacuum is a deeper vacuum (e.g., lower pressure), additional force may be applied to the diaphragm and assist the driver. As a result, the braking force may be increased as the vacuum amount increases.
However, engine displacements are being reduced in an effort to reduce fuel consumption. Smaller displacement engines may have lower frictional losses and they may operate more frequently with less pumping losses than larger displacement engine that are operated at part throttle conditions. However, it may be more difficult to provide lower vacuum levels and large amounts of vacuum with a smaller displacement engine. Consequently, a driver of a vehicle having a smaller displacement engine may apply brakes and receive vacuum assistance such that braking assistance is available to provide only a portion of a desired or requested braking force during vehicle braking. If the driver attempts to increase braking force via further depressing the brake pedal, the driver may notice that less braking force is applied as brake pedal force increases. Thus, the driver may notice that braking force is not increasing at a same rate as when vacuum is available and that the brake pedal feel has become stiffer. Such conditions may degrade a person's driving experience and impressions of the vehicle.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for applying vehicle brakes, comprising: increasing a brake line fluid pressure in response to brake pedal force at a first rate while brake booster working chamber vacuum is a threshold pressure away from atmospheric pressure, and increasing the brake line fluid pressure at the first rate while brake booster vacuum is within the threshold pressure of atmospheric pressure.
By increasing brake line fluid pressure proportionate to brake pedal force whether or not vacuum is available to a brake booster working chamber, it may be possible to improve a driver's perception of vehicle brake application. For example, brake line fluid pressure may increase at a first rate with respect to brake pedal force when vacuum is available to a brake booster. If additional vacuum is not available as brake pedal force continues to increase, a hydraulic pump may be activated and its output may be adjusted to increase brake line fluid pressure at the first rate so that the absence of additional vacuum is not as noticeable to the driver. In this way, an increase in braking force proportional to brake pedal force may be maintained even in the absence of additional brake booster vacuum.
The present description may provide several advantages. In particular, the approach may provide an improved driving experience via maintain an expected rate of brake application even in the presence of a limited amount of brake booster vacuum. Further, the approach may allow the engine to operate more efficiently since the engine may not have to be transitioned to a vacuum generating mode when brake system vacuum is low. Further still, the approach may make the presence of stiff brake pedal feel less noticeable to a driver.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.